The present invention relates generally to single mode lasers and more particularly to a single mode laser implementation which may accommodate a frequency multiplying material to provide an intracavity doubled single frequency laser.
In the prior art, a variety of single longitudinal mode (hereinafter SLM) lasers have been developed. One particular reason for the interest in SLM lasers resides in the ease with which an SLM laser can be converted to a frequency doubled configuration through the addition of a non-linear material within the laser cavity. In addition, certain problems have been encountered when attempts have been made to convert types of laser other than SLM lasers to the intracavity doubled frequency configuration, as will be seen immediately hereinafter.
The non-linear frequency doubling technique of the prior art has often been used to produce coherent radiation in the visible and ultraviolet spectral region. Acceptable optical conversion efficiency has been achieved in this manner. However, many of these frequency doubled lasers suffer a so called "green noise" problem which limits their usefulness in a number of applications. More specifically, the green noise problem introduces amplitude noise (i.e. variation in the intensity of the output beam at the doubled frequency) which is believed to due to gain competition introduced by the presence of additional modes other than one longitudinal fundamental mode in the laser's resonant cavity in combination with the phenomenon of longitudinal mode coupling through a nonlinear doubling process between the various modes which are present. One popular approach to solving the "green noise" problem is to eliminate the additional modes in the laser light (i.e., use an SLM laser) which excites the non-linear material and thereby eliminate longitudinal mode coupling so as to obtain a single doubled output frequency.
A variety of intracavity doubled single longitudinal mode (SLM) laser systems have been developed in the prior art. One approach in achieving SLM operation is through the use of a ring laser geometry. In a ring laser geometry, spatial holeburning is eliminated by a unidirectional traveling wave. SLM operation is thus achieved in a homogeneous broadened laser system. One example of an intracavity doubled SLM laser is disclosed in U.S. Pat. No. 5,052,815, issued Oct. 1, 1991 to Nightingale et al. One of the principal drawbacks in using a ring laser geometry is that it is difficult to align and operate. Also, a ring laser is generally more complicated than a simple linear cavity because of the optical diode and reciprocal retardation compensator used. Further, beam pointing stability of a ring laser is usually not as good as that obtained using a linear cavity. Nevertheless, a ring laser is generally believed to be more efficient than a standing wave linear cavity since the traveling wave extracts all the available gain uniformly. However, in an intracavity doubled laser, a ring laser is not necessarily more efficient than a linear cavity simply because more intracavity elements are required in a ring cavity for unidirectional operation. These additional elements yield more intracavity losses in the doubled frequency ring geometry as compared with those in a doubled frequency linear geometry since intracavity doubled laser systems are extremely sensitive to cavity losses. Obviously, more losses result in less doubled power.
Another technique for producing an SLM laser is disclosed by Lukas et al in U.S. Pat. No. 5,164,947, issued on Nov. 17, 1992. In this patent disclosure, a twisted-mode technique is employed to eliminate spatial holeburning so as to obtain SLM operation. The laser cavity comprises an input mirror and an output coupler which define a linear laser cavity. Inside the laser cavity, a lasant rod is inserted between two quarter-wave plates. A polarizer and a nonlinear optical crystal are also included in the laser cavity to define the polarization direction of the fundamental wave and to generate output radiation at twice the frequency of the fundamental wave. The laser mode is circularly polarized in the laser rod, resulting in a standing wave in which the electric field vector rotates through the gain medium and in which there are no standing wave nodes within the gain medium. Spatial holeburning is thus eliminated. However, this approach has its own limitations and is often difficult to implement. First, it relies on having two precise quarter-wave plates inside the cavity. Second, the laser rod has to be non-birefrigent. This requirement restricts the laser to a limited number of laser hosts. Further, as the laser crystal is optically excited, the thermally and stress induced birefringence will introduce spatial holeburning in the gain medium, again resulting in multiple mode operation. From the standpoint of implementation, the complexities of this laser design make it difficult to scale and operate.
A recent patent disclosure, U.S. Pat. No. 5,381,421, issued to Wedekind et al. on Jan. 10, 1995, describes another way to achieve SLM laser operation in a linear laser cavity. In this approach, a Brewster polarizer and a birefrigent material form a Lyot filter which narrows the frequency bandwidth for single longitudinal mode operation. The major inconvenience of this approach resides in its use of a Brewster polarizer. As we know, the Brewster angle is usually greater than 45 degrees and, thus, is not convenient to work with. Also, a Brewster polarizer is not a perfect polarizer in that it only has about a 16% loss for the polarization which is discriminated against. For this reason the Brewster polarizer may not provide loss at levels sufficient to suppress longitudinal modes other than the fundamental. Further, any slight deviation from the Brewster angle will introduce additional insertion loss which may harm frequency doubling and selection. Similarly, another patent disclosure U.S. Pat. No. 5,430,754, issued to Suzuki et al. on Jul. 4, 1995 discloses a Lyot filter formed by an off-axially cut Nd:YVO.sub.4 and a birefrigent material such as KTP to generate SLM radiation. The limitation of this approach is its reliance on strong birefringence and long crystal length to enhance the Poynting vector walkoff. Also, in the case of Nd:YVO.sub.4, which is the preferred mode of operation, the Nd:YVO.sub.4 is cut 43 deg off of the cleavage plane. The fabrication of such an off-axially cut crystal is not trivial and, typically, is accompanied by a low yield. Moreover, in both of these Lyot filter approaches, a plurality of intracavity elements have surfaces substantially normal to the cavity axis. Residual reflections from these surfaces can lead to intracavity etalon and coupled cavity effects resulting in mode-hopping.
The present invention provides a heretofore unseen approach and associated method for producing an SLM laser which eliminates the problems described above and which is suitable for use in intracavity frequency doubled applications.